The present application claims priority under 35 U.S.C. xc2xa7119 of French Patent Application No. 99 01790, filed on Feb. 15, 1999.
1. Field of the Invention
This invention relates to a wavelength filtering self-aligned retroreflecting optical system.
2. Discussion of Background Information
It is common knowledge that alignment of the optical components is critical for the quality of devices that are fitted with the components. Therefore, any self-alignment, i.e. any assembly in which the properties of the luminous flux are somewhat sensitive to the orientation or to the position of one or several components, is required.
Among the self-aligned retroreflecting systems known for a long time, the following can be mentioned for exemplification purposes: the cube corner illustrated in FIG. 1 with which an incident beam 1, 1xe2x80x2 on a reflecting orthogonal trihedron 3 produces a parallel output beam 2, 2xe2x80x2, without regard to the angle of incidence with respect to the diagonal 5 of the cube and the position of the point of incidence 4.
The so-called xe2x80x98cat""s eyexe2x80x99 assembly is also well known, consisting of a convergent optical system 8 with optical axis 9, in the focal plane of which is placed a mirror 10, more or less perpendicular to the axis 9. A collimated incident beam 11, 11xe2x80x2 converges onto the mirror 10, is reflected on the mirror and then diverges in return onto the optical system 8 that produces an output beam 12, 12xe2x80x2, also collimated and parallel to beam 11, 11xe2x80x2. Such a cat""s eye is represented in FIG. 2.
Both systems described previously offer self-alignment of the direction of the output beam 2, 2xe2x80x2 and 12, 12xe2x80x2 on the input beam, respectively 1, 1xe2x80x2 and 11, 11xe2x80x2 in two dimensions, i.e. in all the planes parallel to the direction of the input beams. In certain systems, self-alignment in a single dimension is desirable. In that case, an orthogonal dihedron instead of the trihedron of FIG. 1 or a cylindrical cat""s eye is used i.e. a lens or a cylindrical optical system instead of the spherical optical system 8 shown in FIG. 2. The dihedron ensures self-alignment in the plane perpendicular to its edge and the cylindrical lens in the plane perpendicular to its generatrix. In the parallel plane, both these systems behave like a mirror.
Optical systems carrying out wavelength filtering are also known the most conventional of which, as represented in FIG. 3, consist of a dispersing system 14 placed between an input slot 15 and an output slot 16 and receiving a wide-spectrum luminous flux 17 under an angle of incidence i. The output luminous flux 18 is spread, its direction depending on the wavelength and only beam 19 selected by the output slot 16 is transmitted by the system. It is also possible to provide a two-stage spectral filtering, whereas an intermediate slot 15xe2x80x2, placed between two gratings 14 and 14xe2x80x2 contribute to limit the spurious light transmitted.
The plane perpendicular to the lines of the grating containing the central ray of the incident beam and the central rays of the beams dispersed by the grating is traditionally called xe2x80x98dispersing planexe2x80x99. For each beam, the xe2x80x98transversal planexe2x80x99 shall be defined as the plane perpendicular to the central ray and the xe2x80x98longitudinal planexe2x80x99 shall be defined as the plane perpendicular to the dispersing plane containing the central ray.
A device of the previous art, is represented in FIG. 4, where the orthogonal dihedron 23 has an edge 23xe2x80x2 parallel to the dispersing plane. This arrangement leads to a mirror-like behavior in this latter plane and to a self-aligned behavior in the longitudinal plane.
A wavelength filtering self-aligned retroreflecting optical system can be used for instance for filtering wavelengths in order to realize the external cavity of wavelength tuneable laser or to select a wavelength. The optical system is very schematically represented in the latter application in FIG. 4. The grating 20 receives an output luminous flux from a first monomode optical wave-guide 21 through the lens 22. The grating 20 diffracts it to the orthogonal dihedron 23 whose edge 23xe2x80x2 is parallel to the dispersing plane of the grating and this dihedron returns it. The grating 20 diffracts this flux again, on the return path, in a direction depending on its wavelength. Thus, only the flux at a wavelength corresponding to the position of the second wave-guide 24 is coupled in the latter. These wave-guides can be in particular optic fibers.
It has been noticed that the performances of the latter device depend to a large extent on the accuracy of the orthogonal dihedron. The purpose of this invention is the realization of a self-aligned retroreflecting optical system that enables good tolerances of the properties of its components; in particular as regards the value of the angle of the dihedron thereby involved.
The purpose of the invention is to offer a device that would solve the above mentioned problems while preserving the advantages of the self-aligned systems known, wherein the reflector behaves like a mirror in the dimension of the dispersing plane and has a self-aligned behavior in the dimension of the longitudinal plane.
Another object of the invention is to enable the realization of a two-stage wavelength selective assembly, in which the adjustment of the position of the intermediate slot is made easier.
To this end, the invention relates to a single dimension self-aligned retroreflecting optical system, for wavelength filtering, comprising:
at least one wave guide with one internal face in the focal plane of collimation optics generating a main collimated beam,
a diffraction grating with parallel lines whereas the said lines disperse, in collimated beams, the different wavelengths of the main beam in directions parallel to the dispersing plane that is perpendicular to the lines of the grating,
a reflector system generating self-alignment in the longitudinal plane perpendicular to the dispersing plane.
According to the invention, the reflector consists of a spherical lens and of a single dimension self-aligned reflector system, whereas the said dimension is parallel to the dispersing plane of the grating.
In different embodiments, each exhibiting specific advantages:
it comprises a single fiber transmitting an input luminous flux and collecting the output luminous flux;
it comprises at least two fibers, one transmitting an input luminous flux, the other collecting an output luminous flux;
the single dimension self-aligned reflector system is a reflector dihedron whose edge is perpendicular to the dispersing plane of the grating and is placed in the focal plane of the lens;
the reflector dihedron is a double total internal reflection prism;
a reflector dihedron forms an angle of approximately 90xc2x0;
at least one of both mirrors of the reflector dihedron has reduced lateral sizes;
the self-aligned reflector system consists of two perpendicular mirrors carried on supports that are independent from another;
each mirror is of limited dimension;
the size of the wave guide modes is broader;
the polarization splitter with parallel outputs is inserted between the collimation optics and the grating and the center of the lens is placed in the middle of the space between the output beams of this splitter;
a horizontal edge dihedron leads a quadruple passage on the grating. whereas the light reaches the self-aligned retroreflector after two passages on the grating.
The invention also relates to a monochromator:
In a first embodiment, a wave-guide is passive and forms the input slot and the output slot of the monochromator.
In a second embodiment, a first passive wave-guide forms the input slot of the monochromator and a second passive wave-guide forms its output slot.
The invention also relates to a laser incorporating such a self-aligned reflector system. In such a case, a wave-guide delineates a wavelength tuneable external cavity laser between its extremity and the single dimension self-aligned reflector system.